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Process and Device Simulation of Field Emission Microtrides

Introduction

One of the most beneficial merits of computer-based experiment using TCAD lies
in its capability of a total simulation of the target device. One can consider
its novel fabrication process and its device performance at the same time without
time consuming and expensive experiments using real wafers. Take field emission
microtriodes, for example.

They have been studied for various applications such as field emission displays,
high-power and high-frequency devices having high-operating temperature and
radiation hardness. In order to fabricate the microtriodes, silicon wafer process
technology is preferred because it is suitable in forming a sharp cone structure
as its field emitter(1).

Process Simulation

The electrical performance of the device is very sensitive to the physical
configuration of its structure. Precise and stable fabrication processes are
necessary. In many cases, compatibility with a conventional CMOS process is
also required for the integration of driver circuits around it. One can investigate
and optimize desirable process steps using ATHENA
process simulator. Figure 1 shows simulated process steps for a microtriode,
applying the idea developed for a field emitter array fabrication process(2).
Starting from an n-type silicon bare wafer, masking oxide is deposited and patterned,
then RIE etching is performed to get a base shape of the field emitter. The
first oxidation sharpens the emitter. After forming a silicon nitride cap on
it, the second oxidation is performed to complete the grid insulator and to
adjust the position of grid electrodes. The nitride cap is removed and grid
metal is deposited. The overlying structure on the emitter is removed by lift-off
and RIE etching.

Figure 1. A microtriode fabrication process simulaitons
by ATHENA.

Device Simulation

Once a desirable structure is obtained by ATHENA
process simulation, one can transfer it to ATLAS device
simulation directly. In some cases, remeshing suitable for device simulation
may be needed to obtain a better convergence and that is easily done using an
user-friendly interactive meshing tool DevEdit. In
order to estimate the electrical characteristics of the emitter, Fowler-Nordheim
equation is used. Figure 2 shows electric potential contours and current flow
lines at the anode and grid voltages of 800V and 10V respectively. It can be
seen that the current flow lines respond very sensitively to slight irregularities
in the shape of the structure. The final results of the emitter current vs.
anode voltage curves at various grid voltages are shown in Figure 3.

Figure 2. Current flow lines and potential contours
by ATLAS.

Figure 3. ATLAS simulation results of emitter currents
as a function of anode voltage for different grid voltages.

Conclusion

An field emission microtriode can be simulated continuously starting from a
bare wafer, going through the device structure, to the resultant I-V curves,
using TCAD process and device simulators ATHENA and
ATLAS. Making good use of TCAD total simulations
will reduce process development time and the number of trial experiments.